1. Field of the Invention
The present invention relates to a sensor capable of detecting three-axis accelerations and an angular rate around two axes using a piezoelectric device.
2. Description of the Related Art
In electronic industry, automobile industry and machine industry, demands for the multi-axial accelerometer and angular rate sensor having a small size and a high reliability have been increased. Such sensors are requested to be capable of detecting the acceleration and angular rate of each component of two dimensions or three dimensions.
The multi-axial accelerometer has a weight as disclosed in Japanese Patent 3141954 and this weight is held by a flexible base plate around the weight via the flexible base plates having flexibility and detecting devices are disposed on the base plate. In the accelerometer having such a structure, when acceleration is applied to the sensor, the weight is displaced receiving a force proportional to the acceleration so that a mechanical deformation is generated in the flexible base plate. In the sensor, the detecting devices provided on the flexible base plate detect a deformation of the flexible base plate generated accompanied by the acceleration so as to detect the acceleration. In the accelerometer having the piezoresistive device as its detecting device, this mechanical deformation of the flexible base plate is detected as a change in resistance in the piezoresistive device so as to detect the acceleration. Further, there has been known another accelerometer provided with a capacitance type device, having a structure in which electrode plates are disposed such that a electrode plate is opposed to another electrode plate provided on the flexible base plate as other detecting device than the piezoresistive device in order to detect the mechanical deformation of the flexible base plate as a change in capacitance between the electrode plates. According to Japanese Patent 3141954, a piezoelectric device in which piezoelectric membrane is sandwiched between two electrode plates is provided on the flexible base plate so as to detect the mechanical deformation of the flexible base plate as a change in charges generated in the piezoelectric device.
The multi-axial angular rate sensor has the same structure as the above-described multi-axial accelerometer and further an exciting device for applying vibration to the weight. In the angular rate sensor, the weight is vibrated cyclically in a specific axial direction by the exciting device. When an angular rate is generated around an axis perpendicular to the vibration axis of the weight during a vibration of the weight, a Coriolis force is applied in an axial direction perpendicular to these two axes. The weight is displaced by this Coriolis force, so that the flexible base plate connected to the weight is deformed. This mechanical deformation of the flexible base plate is detected by the detecting device so as to detect the angular rate. Japanese Patent 3534251 has disclosed an angular rate sensor for detecting the Coriolis force with a device for vibrating the flexible base plate with electrostatic force generated when a voltage is applied in between electrodes, a device for detecting changes in capacitance by vibrating the weight by means of a piezoelectric device for vibrating the base plate due to piezoelectric effect, a piezoresistive device, and the piezoelectric device.
However, in the sensor having the aforementioned piezoresistive device, its detection value contains an error if the temperature of ambient environment is changed, because the piezoresistive has temperature dependency. Thus, compensation of the temperature is necessary for an accurate measurement. Particularly in the case where the sensor is used in automobile industry, there occurs such a problem that the temperature compensation is required in a wide operating temperature range of −40° C. to 120° C.
Although the sensor for detecting the aforementioned changes in capacitance has an advantage that the manufacturing cost is low, it can hardly execute signal processing because a formed capacitance is small.
The aforementioned sensor having the piezoelectric device is advantageous in that its detection sensitivity is high because it generates an electromotive force directly. However, piezoelectric zirconate titanate (PZT) needs to be used as a piezoelectric device, thereby posing a problem that the piezoelectric constant of the PZT is highly dependent on the temperature.
The multi-axial accelerometer and angular rate sensor of prior art have problems that their temperature dependency is high or they are difficult to manufacture. Thus, the multi-axial accelerometer and angular rate sensor have been demanded to be of a smaller size and capable of executing high accuracy detection without any temperature compensation and be easy to manufacture.